Some patients requiring respiratory assistance may be placed on what is known as a ventilator. The ventilator is typically connected through an endotracheal tube to the patient. A mixture of oxygen and air is periodically applied under pressure to force inhalation or inspiration by the patient. Exhalation is generally voluntary.
It is not desired for the patient to rest for too long a period between exhalation and inhalation. The forced inhalation is provided by a timer, which may be a part of a computer, so that inhalation is forced on a regular timed basis at a relatively short assumed time after the end of exhalation. Since the exhalation is generally voluntary, it is difficult to tell precisely when it will end, and thus when inhalation should start. It is known that expiration typically occurs approximately on a 2 to 1 time basis relative to inhalation. However, if the patient takes longer than assumed to complete exhalation, it is possible that a forced inspiration could start before exhalation was complete. This results in air entrapment or "stacking", and over a period of time enough air could be entrapped or stacked in a patient's lungs to cause blowing up or exploding of the patient's lungs with a severe danger of patient death.